1. Technical Field
The present invention relates to processing audio signals, more particularly, to processing audio signals reproducing sound on virtual channels.
2. Description of the Related Art
Audio plays a significant role in providing a content rich multimedia experience in consumer electronics. The scalability and mobility of consumer electronic devices along with the growth of wireless connectivity provides users with instant access to content. FIG. 1a illustrates a conventional audio reproduction system 10 for playback over headphones 12 or a loudspeaker 14 that is well understood by those skilled in the art.
A conventional audio reproduction system 10 receives digital or analog audio source signal 16 from various audio or audio/video sources 18, such as a CD player, a TV tuner, a handheld media player, or the like. The audio reproduction system 10 may be a home theater receiver or an automotive audio system dedicated to the selection, processing, and routing of broadcast audio and/or video signals. Alternatively, the audio reproduction system 10 and one or several audio signal sources may be incorporated together in a consumer electronics device, such as a portable media player, a TV set, a laptop computer, or the like.
An audio output signal 20 is generally processed and output for playback over a speaker system. Such output signals 20 may be two-channel signals sent to headphones 12 or a pair of frontal loudspeakers 14, or multi-channel signals for surround sound playback. For surround sound playback, the audio reproduction system 10 may include a multichannel decoder as described in U.S. Pat. No. 5,974,380 assigned to Digital Theater Systems, Inc. (DTS) hereby incorporated herein by reference. Other commonly used multichannel decoders include DTS-HD® and Dolby® AC3.
The audio reproduction system 10 further includes standard processing equipment (not shown) such as analog-to-digital converters for connecting analog audio sources, or digital audio input interfaces. The audio reproduction system 10 may include a digital signal processor for processing audio signals, as well as digital-to-analog converters and signal amplifiers for converting the processed output signals to electrical signals sent to the transducers (headphones 12 or loudspeakers 14).
Generally, loudspeakers 14 may be arranged in a variety of configurations as determined by various applications. Loudspeakers 14 may be stand alone speakers as depicted in FIG. 1a. Alternatively, loudspeakers 14 may be incorporated in the same device, as in the case of consumer electronics such as a television set, laptop computers, hand held stereos, or the like. FIG. 1b illustrates a laptop computer 22 having two encased speakers 24a, 24b positioned parallel to each other. The encased speakers are narrowly spaced apart from each other as indicated by a′. Consumer electronics may include encased speakers 24a, 24b arranged in various orientations such as side by side, or top and bottom. The spacing and sizing of the encased speakers 24a, 24b are application specific, thus dependent upon the size and physical constraints of the casing.
Due to technical and physical constraints, oftentimes audio playback is compromised or limited in such devices. This is particularly evident in electronic devices having physical constraints where speakers are narrowly spaced apart, or where headphones are utilized to playback sound, such as in laptops, MP3 players, mobile phones and the like. Some devices are limited due to the physical separation between speakers and because of a correspondingly small angle between the speakers and the listener. In such sound systems the width of the perceived sound stage is generally perceived by the listener as inferior to that of systems having adequately spaced speakers. Oftentimes product designers abstain from deviating from a television's aesthetic design by not including a center mounted speaker. This compromise may limit the overall sound quality of the television as speech and dialogue are directed to the center speaker.
To address these audio constraints, audio processing methods are commonly used for reproducing two-channel or multi-channel audio signals over a pair of headphones or a pair of loudspeakers. Such methods include compelling spatial enhancement effects to improve the audio playback in applications having narrowly spaced speakers.
In U.S. Pat. No. 5,671,287, Gerzon discloses a pseudo-stereo or directional dispersion effect with both low “phasiness” and a substantially flat reproduced total energy response. The pseudo-stereo effect includes minimal unpleasant and undesirable subjective side effects. It can also provide simple methods of controlling the various parameters of a pseudo-stereo effect such as the size of angular spread of sound sources.
In U.S. Pat. No. 6,370,256, McGrath discloses a Head Related Transfer Function on an input audio signal in a head tracked listening environment including a series of principle component filters attached to the input audio signal and each outputting a predetermined simulated sound arrival; a series of delay elements each attached to a corresponding one of the principle component filters and delaying the output of the filter by a variable amount depending on a delay input so as to produce a filter delay output; a summation means interconnected to the series of delay elements and summing the filter delay outputs to produce an audio speaker output signal; head track parameter mapping unit having a current orientation signal input and interconnected to each of the series of delay elements so as to provide the delay inputs.
In U.S. Pat. No. 6,574,649, McGrath discloses an efficient convolution technique for spatial enhancement. The time domain output adds various spatial effects to the input signals using low processing power.
Conventional spatial audio enhancement effects include processing audio signals to provide the perception that they are output from virtual speakers thereby having an outside the head effect (in headphone playback), or beyond the loudspeaker arc effect (in loudspeaker playback). Such “virtualization” processing is particularly effective for audio signals containing a majority of lateral (or ‘hard-panned’) sounds. However, when audio signals contain center-panned sound components, the perceived position of center-panned sound components remains ‘anchored’ at the center-point of the loudspeakers. When such sounds are reproduced over headphones, they are often perceived as being elevated and may produce an undesirable “in the head” audio experience.
Virtual audio effects are less compelling for audio material that is less aggressively mixed for two-channel or stereo signals. In this regard, the center-panned components dominate the mix, resulting in minimal spatial enhancement. In an extreme case where the input signal is fully monophonic (identical in the left and right audio source channels), no spatial effect is heard at all when spatial enhancement algorithms are enabled.
This is particularly problematic in systems where loudspeakers are below a listener's ear level (horizontal listening plane). Such configurations are present in laptop computers or mobile devices. In these cases, the processed hard-panned components of the audio mix may be perceived beyond the loudspeakers and elevated above the plane of the loudspeakers, while the center-panned and/or monophonic content is perceived to originate from between the original loudspeakers. This results in a very ‘disjointed’ reproduced stereo image.
Therefore, in view of the ever increasing interest and utilization of providing spatial effects in audio signals, there is a need in the art for improved virtual audio processing.